1. Field of the Invention
The present invention relates to apparatus and methods for thermomechanically treating sintered and heat treated powder metal steel gears in the metastable austenitic condition to produce increased surface densification and plastic deformation of the load bearing surfaces and, thereby, higher strength and accurate contact surfaces through a net shape finishing process. The invention particularly relates to an apparatus and method for increasing the bending strength and surface durability of powder metal gears by ausforming the tooth surface and sub-surface layers of gears including spur, helical and intersecting axis type gears. Applications for ausformed powder metal gears include automotive power transmissions in the operation of which the gears are subjected to high loads and speeds.
2. Description of the Prior Art
The sintered powder metal process is making inroads in many vehicle applications because of the substantially lower costs of powder metal steel components for high volume production. However, because of current strength limitations of powder metal components, applications in vehicle power transmissions have been limited only to lower-loaded components. Although powder metal gears are increasingly being used in powered hand tools, gear pumps, and as accessory components in automotive transmissions, powder metal gears have not been used for power transmission gearing. The state-of-the-art powder metal gears do not possess adequate tooth bending strength and pitting/wear resistance as compared to gears produced from wrought and/or forged steels.
Highly loaded gears used for power transmission gearing are conventionally manufactured from wrought and/or forged low carbon, low to medium alloyed steel blanks. After preliminary blank machining, gear teeth are produced by metal cutting operations such as hobbing or shaping, or by forging to near net shape. Gears are then heat treated to impart desired surface strength, strength gradient and core toughness. Heat treatment involves carburizing the surface of low carbon steel gears to increase the surface and near surface carbon content, followed by hardening by rapid quenching to below the temperature at which a diffusionless transformation process that creates a hardened martensitic structure proceeds to completion, the so-called Ms temperature. Alternatively, gear wheels produced from medium to high carbon alloy steel compositions, which therefore do not need carburization, are instead induction hardened, wherein only the gear tooth surfaces are heated and then quenched to produce the hardened martensitic structure. The hardened gears are then finished to net shape by grinding, skiving, burnishing, and/or honing operations.
For powder metal gears, a method has been described in U.S. Pat. No. 5,711,187, wherein a powder metal gear wheel formed from a pressed and sintered powder metal blank is claimed to be surface hardened by densifying the tooth surface layers, both in the flank and root/fillet region. The '187 patent describes a pre-finishing technique of gear rolling that is performed prior to heat treatment and hardening using either a single-die or double-die rolling apparatus, and is applicable for sintered low alloy steel compositions similar to SAE 4100, SAE 4600, and SAE 8600 grades. However, as the method described in the '187 patent is a pre-finishing operation that is performed prior to heat treatment and hardening, it is applicable only to low carbon low alloy sintered steel compositions in the soft machinable condition, particularly compositions with carbon contents of 0.2% or less. The '187 patent claims full theoretical densification at the rolled surfaces and a progressively decreasing densification (90-100%) gradient of at least 380 microns up to about 1000 microns in depth.
As noted above, the method described in the '187 patent is applicable only to relatively soft gear wheel blanks made of low carbon low alloy sintered powder metal steel compositions with hardness typically less than BHN 180 (or HRC less than 24). Gear rolling of soft sintered gear tooth surfaces as described in '187 produces densification of tooth surface layers. As has been noted above, powder metal gears, either in the as-sintered condition or after surface densification by gear rolling as described in the '187 patent, have to be heat treated by carburizing and hardening operations to achieve the specific surface hardness, hardness gradient and core strength necessary for high load bearing power transmission gearing. Any surface hardening achieved due to work hardening by gear rolling and related surface densification as described in '187 is substantially eliminated during the subsequent heat treatment process.
Furthermore, because the sintered and densified powder metal gears produced by the method described in '187 are subjected to heat treatment and hardening, the gears may require subsequent hard finishing by grinding, skiving, burnishing or honing operations to achieve the required level of accuracy, resulting in removal of about 150 microns of the densified surface region of gear teeth. This removal of the portion of the surface region with improved apparent hardness of powder metal densified surface layers lowers the load bearing capacity.
Apparatus and methods have been described in U.S. Pat. Nos. 5,221,513; 5,391,862; 5,451,275; 5,656,106; 5,799,398; 6,007,762; 6,126,892 for wrought and/or forged steel gear wheels and U.S. Pat. No. 6,264,768 for rolling element bearings in which a carburized and hardened workpiece is finished by thermomechanical means by inducing controlled plastic deformation in the metastable austenitic condition via gear rolling. Such a thermomechanical treatment, also called ausform finishing, of hardened gear tooth surfaces involves reaustenitization by induction heating followed by marquenching at about 450-500° F. or above the start of the martensite transformation temperature, the so-called Ms temperature. The gear teeth in this marquenched condition are roll finished and then finally quenched to martensite before any diffusional decomposition can form from the metastable austenite. For wrought and/or forged gear wheels, the thermomechanical method of ausform finishing described in the above-identified patents results in substantial material flow up and down the tooth surfaces and in the axial direction due to combined rolling and sliding action on the tooth surfaces. Unlike conventional gear finishing such as grinding, the outermost surface hardened layers are not removed during the ausform finishing operation.
The method described in the previously mentioned patents is also applicable to medium to high carbon alloyed gear steels, wherein the carbon content is sufficiently high such that the carburizing operation is not required. The thermomechanical procedure described in the patents is thus applicable to both low carbon carburized/hardened gear steels as well as medium to high carbon induction hardenable gear steels and is employed in the present invention.